Semiconductor laser element, semiconductor etchant, and method of fabricating the semiconductor laser element

ABSTRACT

The semiconductor laser element comprises, from bottom to top, the p-Al x Ga 1-x As upper clad layer, p-Al y Ga 1-y As resistance control layer, and p-GaAs cap layer (where x&gt;y&gt;0.2). A portion of only the resistance control layer and cap layer is selectively etched. The etchant used for this etching is a mixture of organic acid and hydrogen peroxide based mixture, has such a composition such that the ratio of dissolution rate of the upper clad layer to the cap layer is between 10 and 20, and pH is between 7.4 and 7.6.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser element having acurrent non-injection region. This invention also relates to asemiconductor etchant (“etchant”) used for selectively etching acompound semiconductor crystal, and a method of fabricating thesemiconductor laser element using the semiconductor etchant.

BACKGROUND OF THE INVENTION

GaAs-based semiconductor laser elements are widely used in excitationlight sources of optical amplifiers and the like. When the GaAs-basedsemiconductor laser element is used in excitation light sources, it isnecessary that its light output is high. However, when the light outputof the semiconductor laser element is increased, following phenomenondisadvantageously occur at the laser end facet of the semiconductorlaser element. Firstly there occurs an optical damage, secondly thereoccurs a corrosion of the laser end facet when the laser element isoperated over a long period. It is believed that these phenomenon arecaused because of increase of the temperature of the end facet(resonator surface), contraction of the band gap, photo-absorption,recombination current, and a combination of one or more of these.

When the light output of the semiconductor laser element is increased,the optical damage and end facet corrosion become more conspicuousbecause the light density at the end facet increases. Sometimes thedeterioration is so high that the generation of laser is suddenlystopped. In order to overcome these problems, it is desirable to have asemiconductor laser element in which light intensity is reduced onlynear the end facet.

As a countermeasure, Japanese Patent Application Laid-Open No. 6-188511discloses a semiconductor laser element which has resonator end facetswith different reflectances, and a ridge mesa on an active layer. Theridge mesa is formed in the region except in a region near the resonatorend facet on the low reflectance side. At least a part of the regionwhere the ridge mesa is formed is provided with a current non-injectionstructure.

A cross-section of the semiconductor laser element proposed in theabove-mentioned reference is shown in FIG. 9. A cross section of thesemiconductor laser element along the line A—A shown in FIG. 9 is shownin FIG. 10. This semiconductor laser element is fabricated by thefollowing method.

1) An epi-wafer is fabricated by stacking a plurality of layer on n-GaAssubstrate 1. In this epi-wafer, n-GaAs (n=1×10¹⁸ cm⁻³) buffer layer 2 ofthickness 0.5 μm, n-AlGaAs (n=1×10¹⁸ cm⁻³) lower clad layer 3 ofthickness 1.5 μm, n-GaAs (n=3×10¹⁷ cm⁻³) lower optical confinement layer4 of thickness 0.03 μm, p-In_(0.2)Ga_(0.8)As (p=3×10¹⁷ cm⁻³) activelayer 5 of thickness 80 Å, p-GaAs (p=3×10¹⁷ cm⁻³) upper opticalconfinement layer 6 of thickness 0.03 μm, p-Al_(0.35)Ga_(0.65)As(p=1×10¹⁸ cm⁻³) upper clad layer 7 of thickness 1.2 μm, and p-GaAs(p=4×10¹⁹ cm⁻³) cap layer 9 of thickness 0.5 μm are successively stackedon the n-GaAs substrate 1.

2) A ridge mesa having a width of about 2 to 3 μm and length of about800 μm is created on the epi-wafer using photolithography technique. Asa result, length of the cavity of this semiconductor laser elementbecomes 800 μm.

3) The cap layer 9 from the anti-reflection side end facet, that is,from the laser-emission side end facet F1 is to a width of 25 μm isremoved by selective etching to obtain the current non-injectionstructure. The reference numeral F2 denotes the laser-reflection sideend facet.

The cap layer may be removed using the semiconductor etchant disclosed,for example, in Japanese Patent Application Laid-Open No. 7-7004. Thisreference discloses an etchant that selectively etches only the GaAslayer when there exists layers of GaAs and AlGaAs. This method istherefore called selective etching. The etchant is prepared by adding abasic compound to a mixture of organic acid and hydrogen peroxide basedmixture in such a manner that the pH of the mixture is between 6.0 and8.0. The organic acid is, for example, citric acid.

Precisely, aqueous citric acid solution (1% by weight) and aqueoussolution of hydrogen peroxide solution (30% by weight) are mixed at avolume ratio of 100:1. Ammonia is added to this mixture in such a mannerthat the pH of the mixture is between 6.0 and 8.0. Assume that the ratioof the etching rate of the p-GaAs cap layer 9 to that of thep-Al_(0.35)Ga_(0.65)As upper clad layer 7 is called as selection ratio.Then, the p-GaAs cap layer 9 is etched more effectively when theselection ratio is high and etching can be stopped exactly at thep-Al_(0.35)Ga_(0.65)As upper clad layer 7. Based on an experiment it wasconfirmed that the selection ratio is 85 when the pH of the etchant is7.0. In an another experiment the p-GaAs cap layer 9 was removed usingthe etchant having the pH 7.0.

4) Both the surfaces of the ridge mesa were then covered with the SiNfilm 10. Finally, p-electrode 11 and n-electrode 12 are stacked to havethe semiconductor laser element. This semiconductor laser element isalso called a ridge waveguide type semiconductor laser element.

FIG. 11 shows a cross of another conventional semiconductor laserelement. The difference between the semiconductor laser element shown inFIG. 11 and that shown in FIG. 9 is that the semiconductor laser elementshown in FIG. 11 has p-Al_(0.15)Ga_(0.85)As resistance control layer 8formed between the p-GaAs cap layer 9 and the p-Al_(0.35)Ga_(0.65)Asupper clad layer 7. It is known that the resistivity of the p-GaAs caplayer 9 can be reduced by employing such a structure. FIG. 12 shows across section of the semiconductor laser element along the line A-Ashown in FIG. 11. The current non-injection structure is formed byselectively etching a region from the laser-emission side end facet ofthe cap layer 9 and the resistance control layer 8. The width of thisremoved region is 25 μm. In other words, in this semiconductor laserelement it is necessary to etch both the p-GaAs cap layer 9 and thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 in one etching.

However, when both the p-GaAs cap layer 9 and the p-Al_(y)Ga_(1-y)Aslayer 8 are to be removed by one etching, depending on the compositionratio y of aluminum, a small portion of p-Al_(y)Ga_(1-y)As resistancecontrol layer 8 is disadvantageously leftover as it is without beingetched.

If even a small portion of the p-Al_(y)Ga_(1-y)As resistance controllayer 8 remains then the end facet corrosion occurs when the laseroutput is increased. FIG. 11 and FIG. 12 show a portion 8 a, of thelayer 8, that remained above the upper clad layer 7 because ofincomplete etching. When this portion 8 a remains above the upper cladlayer 7, which is a current non-injection layer, injection current flowsinto the laser-emission side end facet of the active layer 5 throughthis non-etched region 8 a. When current flows in the active layer 5,the above-mentioned cycle of positive feedback occurs, and opticaldamage and end facet corrosion occur at the laser-emission side endfacet.

Thus, with the conventional technology, it is possible to effectivelyetch only the GaAs layer when there are layers of GaAs and AlGaAs.However, it is almost impossible to completely etch the p-GaAs layer andthe p-Al_(y)Ga_(1-y)As layer in one etching when there are layers ofp-GaAs, p-Al_(y)Ga_(1-y)As, and p-Al_(x)Ga_(1-x)As.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductorlaser element having an improved current non-injection structure. It isanother aspect of this invention to provide an etchant which canselectively and completely etch desired layers in one etching withoutharming the other layers. It is still another aspect of this inventionto provide a method of fabricating the semiconductor laser elementaccording to the present invention using the etchant according to thepresent invention.

According to the semiconductor laser element of one aspect of thisinvention, desired layers are completely etched without harming otherlayers. Furthermore, an insulating layer that has substantially the samethickness as the thickness of the etched layers is formed in the portionfrom where the layers are etched. The layers may be etched only near thelaser-emission side end facet, or maybe etched near both thelaser-emission side end facet and the laser-reflection side end facet.

According to the semiconductor laser element of another aspect of thisinvention, desired layers are completely etched without harming otherlayers. Furthermore, an insulating layer that covers only the surfacesof the layers that were exposed due to the etching is formed.

According to the etchant of still another aspect of this invention, whenthere are first, second, and third layers from top to bottom in order ina semiconductor laser element, then an etchant for which the ratio ofdissolution rates of the first semiconductor layer to the thirdsemiconductor layer is between 10 and 20 is used.

Other objects and features of this invention will become apparent fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a semiconductor laser element accordingto a first embodiment of this invention.

FIG. 2 shows a cross section of the semiconductor laser element shownFIG. 1 along the line A—A.

FIG. 3A to FIG. 3D show how the semiconductor laser element shown inFIG. 1 is fabricated.

FIG. 4 is a graph that shows a relation between the pH of etchant andthe selection ratio.

FIG. 5 shows a cross section of a semiconductor laser element accordingto a second embodiment of this invention.

FIG. 6 is a table that shows a comparison between the semiconductorlaser elements formed using the etchant according to the first andsecond embodiments and the semiconductor laser element formed using theconventional etchant.

FIG. 7 shows a cross section of a semiconductor laser element accordingto a third embodiment.

FIG. 8A to FIG. 8D show how the semiconductor laser element shown inFIG. 7 is fabricated.

FIG. 9 shows a cross section of a conventional semiconductor laserelement.

FIG. 10 shows a cross section of the semiconductor laser element shownin FIG. 9 along the line A—A.

FIG. 11 shows a cross section of another conventional semiconductorlaser element which has a resistance control layer.

FIG. 12 shows a cross section of the semiconductor laser element shownin FIG. 11 along the line A—A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Three embodiments of the present invention will be explained in detailbelow with reference to the accompanying drawings. However, thisinvention should by no means limited to these embodiments.

A cross of the semiconductor laser element according to a firstembodiment of this invention is shown in FIG. 1. A cross section of thesemiconductor laser element shown in FIG. 1 along the line A—A is shownin FIG. 2. The semiconductor laser element according to the firstembodiment is fabricated in the manner as shown in FIG. 3A to FIG. 3D.

The semiconductor laser element according to the first embodiment hasthe same structure as that of the conventional semiconductor laserelement shown in FIG. 11 and FIG. 12. The difference between the two isthat, current non-injection structure can surely be formed in thesemiconductor laser element according to the first embodiment. This hasbecome possible because of the use of an etchant according to thisinvention which completely etches the desired layers.

The semiconductor laser element according to the first embodiment isfabricated in the manner explained below. To begin with, as shown inFIG. 1 and FIG. 2, the n-GaAs substrate 1 is formed. Then, the n-GaAs(n=1×10¹⁸ cm⁻³) buffer layer 2 of thickness 0.5 μm, n-AlGaAs (n=1×10¹⁸cm⁻³) lower clad layer 3 of thickness 1.5 μm, n-GaAs (n=3×10¹⁷ cm⁻³)lower optical confinement layer 4 of thickness 0.03 μm,p-In_(0.2)Ga_(0.8)As (p=3×10¹⁷ cm⁻³) active layer 5 of thickness 80 Å,p-GaAs (p=3×10¹⁷ cm⁻³) upper optical confinement layer 6 of thickness0.03 μm, and p-Al_(0.35)Ga_(0.65)As (p=1×10¹⁸ cm⁻³) upper clad layer 7of thickness 1.2 μm are successively stacked above the n-GaAs substrate1.

Subsequently, as shown in FIG. 3A in more detail, thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 of thickness 0.5 μmand the p-GaAs (p=4×10¹⁹ cm⁻³) cap layer 9 of thickness 0.03 μm aresuccessively stacked above the upper clad layer 7. Then, as shown inFIG. 3B, a portion of the cap layer 9 and resistance control layer 8beginning form the laser-emission side end facet F1 is completelyremoved using an etchant. The width of the etched portion is 25 μm. As aresult, top surface of the upper clad layer 7 in a portion E near thelaser-emission side end facet F1 is exposed. The reference symbol F2denotes the laser-reflection side end facet.

Thereafter, as shown in FIG. 3C, SiN film 10 is formed in the portionfrom where the cap layer 9 and the resistance control layer 8 wereremoved. At this time, the upper clad layer 7, the resistance controllayer 8, and the cap layer 9 are etched into a mesa stripe, and the SiNfilm 10 is also formed around the periphery of the etched area. Then,the p-electrode 11 and n-electrode 12 are formed to have the ridgewaveguide type semiconductor laser element. Finally, a coating of ananti-reflection film is created on the laser-emission side end facet F1,and a coating of optical reflection film is created on thelaser-reflection side end facet F2.

The etchant used for selectively etching the cap layer 9 and resistancecontrol layer 8 is prepared as follows. Aqueous citric acid solution (1%by weight) and aqueous hydrogen peroxide solution (30% by weight) aremixed in a volume ratio of 100:1. The pH of this mixture is adjusted tobe 7.5 by adding ammonia to the mixture. The selection ration, i.e. theratio of the etching rate of the p-GaAs layer to that of thep-Al_(0.35)Ga_(0.65)As layer, is about 18 in this case. The obtainedetchant will be referred to as etchant T1. The etchant T1 is used toselectively etch only the p-GaAs cap layer 9 and p-Al_(0.15)Ga_(0.85)Asresistance control layer 8 without harming the p-Al_(0.35)Ga_(0.65)Asupper clad layer 7. The p-Al_(0.15)Ga_(0.85)As resistance control layer8 is completely removed in this etching.

The etchant is not limited to the one described above. Another etchantwill be explained here. This etchant is obtained by mixing aqueouscitric acid solution (1% by weight) and aqueous hydrogen peroxidesolution (30% by weight) in a volume ratio of 100:1. The pH of thismixture is adjusted to be 6.0 by adding ammonia to the mixture. Theselection ratio in this case is about 10. The obtained etchant will bereferred to as etchant T2.

The relation between the pH of the etchant and the selection ratio willbe explained below with reference to FIG. 4. After numerous experiments,it was confirmed that the p-GaAs cap layer 9 and the p-Al_(y)Ga_(1-y)Asresistance control layer 8 can be completely etched and thep-Al_(x)Ga_(1-x)As upper clad layer 7 remains unharmed when theselection ratio is between 10 and 20. From FIG. 4 it can be seen that,the selection ratio will be between 10 and 20 when the pH of the etchantis between 6.0 to 6.1 or between 7.4 to 7.6. The selection ratio to pHcurve is steeper when the pH is between 6.0 and 6.1. The selection ratioto pH curve is gentle when the pH is between 7.4 and 7.6. This meansthat, stable and effective etching can be performed when the pH of theetchant is between 7.4 to 7.6. Therefore, the pH of the etchant shouldpreferably be between 7.4 and 7.6. However, better results will beobtained when the pH is between 7.4 and 7.8.

According to the first embodiment, the pH of citric acid based etchantis set between 6.0 to 6.1 or between 7.4 to 7.6 and this etchant is usedfor the selective etching to obtain the semiconductor laser element. Asa result, only the p-GaAs cap layer 9 and p-Al_(0.5)Ga_(0.85)Asresistance control layer 8 are effectively etched without harming thep-Al_(0.35)Ga_(0.65)As upper clad layer 7. The obtained semiconductorlaser element has an advantage that the light output is high, intensityof light is low near the end facet, and there is the p-Al_(y)Ga_(1-y)Aslayer which reduces resistivity of the p-GaAs layer. Better results areachieved when the pH of the etchant is between 7.4 to 7.6.

In the first embodiment, a case is described in which the currentnon-injection structure is formed only on the laser-emission side endfacet F1. However, the current non-injection structure may be formed onboth the laser-emission side end facet F1 and the laser-reflection sideend facet F2. When the current non-injection structure is formed on thelaser-reflection side end facet F2, optical damage or end facetcorrosion that may occur at the laser-reflection side end facet F2 canreliably be prevented. A case in which the current non-injectionstructure is formed on both the laser-emission side end facet F1 and thelaser-reflection side end facet F2 will be explained as a secondembodiment.

FIG. 5 shows a cross section of the semiconductor laser elementaccording to the second embodiment. SiO₂ film 20 as insulating film isprovided near the laser-emission side end facet F1. Similarly, SiO₂ film21 as insulating film is provided near the laser-reflection side endfacet F2. The insulating SiO₂ films 20 and 21 correspond to the SiN filmin the first embodiment. In other words, current non-injection structureis provided on both the laser-emission side end facet F1 and thelaser-reflection side end facet F2. The depth of the currentnon-injection structure from the respective end facet is 25 μm.

The etchant used for selectively etching the cap layer 9 and resistancecontrol layer 8 is prepared as follows. Ammonia is mixed with aqueouscitric acid solution (1% by weight) to adjust the pH to 7.4. Then thismixture and aqueous hydrogen peroxide solution (30% by weight) are mixedin a volume ratio of 50:1. The pH of the resultant mixture is adjustedto be 7.4. The selection ratio in this case is about 15. The obtainedetchant will be referred to as etchant T3.

The etchant T3 is used to selectively etch only the p-GaAs cap layer 9and p-Al_(0.15)Ga_(0.85)As resistance control layer 8 without harmingthe p-Al_(0.35)Ga_(0.65)As upper clad layer 7. Thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 was completely removedin this etching.

Then, the p-electrode 11 and n-electrode 12 are formed in the manner asalready explained in the first embodiment. Finally, a coating of ananti-reflection film is created on the laser-emission side end facet F1,and a coating of optical reflection film is created on thelaser-reflection side end facet F2 in the manner as already explained inthe first embodiment.

The etchant is not limited to the one described above. Another etchantwill be explained here. Ammonia is mixed with aqueous citric acidsolution (1% by weight) to adjust the pH to 6.1. Then this mixture andaqueous hydrogen peroxide solution (30% by weight) are mixed in a volumeratio of 50:1. The pH of the resultant mixture is adjusted to be 6.1.The selection ratio in this case is about 15. The obtained etchant willbe referred to as etchant T4.

The result of comparison between the etchants T1 and T2 used in thefirst embodiment, etchants T3 and T4 used in the second embodiment, andthe conventional etchant will be explained below with reference to FIG.6. Just for reference, the pH of the conventional etchant is 7.0 and theselection ratio is 85.

Semiconductor laser elements were formed using the etchants T1, T2, T3,T4, and conventional etchant. Horizontal size of near-field pattern(NFP), and the rise ratio (ΔI_(op)) of threshold current after thereliability test at 100 mW, 50° C. for 1000 hours of these semiconductorlaser elements were measured. The results are shown in FIG. 6.

The NFP size of the semiconductor laser elements obtained by using theetchants T1, T2, T3, T4 according to the present invention is broaderthan that of the semiconductor laser elements obtained by using theconventional etchant. Moreover, the rise ratio ΔI_(op) of the thresholdcurrent is less in the semiconductor laser elements according to thepresent invention than that in the conventional semiconductor laserelement. This means that the light intensity near the laser-emissionside end facet F1 of the semiconductor laser elements according to thepresent invention is less than that of the conventional semiconductorlaser element. In other words, there will be less deterioration of theend facet of the semiconductor laser elements according to the presentinvention than that of the conventional semiconductor laser element.

In the first and second embodiments a case is explained in which aninsulating film is filled in the portion from where the layers wereremoved by etching. However, the portion from where the layers wereremoved by etching may be covered (coated) with an insulating. This casewill be described below as a third embodiment.

Cross section of the semiconductor laser element according to the thirdembodiment is shown in FIG. 7. The number and composition of the layersbelow the layer 7 shown in FIG. 7 is the same as those shown in FIG. 1or FIG. 5, therefore, their explanation will be omitted to avoid simplerepetition of explanation.

The differences between the semiconductor laser element shown in FIG. 7and that shown in FIG. 5 are as follows. Insulating SiN films 32 a and32 b are provided instead of the insulating SiO₂ films 20 and 21. TheSIN films 32 a and 32 b cover the surface of the lower clad layer 7 thatis exposed because of etching. Furthermore, the p-electrode 11 coversthe SiN films 32 a and 32 b and the cap layer 9.

Thus, current non-injection structure is provided on both thelaser-emission side end facet F1 and the laser-reflection side end facetF2. The depth of the current non-injection structure from the respectiveend facet is 25 μm.

The semiconductor laser element according to the third embodiment isfabricated in the manner as shown in FIG. 8A to FIG. 8D. The layers fromthe n-GaAs substrate 1 to the p-Al_(0.35)Ga_(0.65)As upper clad layer 7are stacked in the same manner as explained in the first embodiment.Subsequently, as shown in FIG. 8A, p-Al_(0.15)Ga_(0.85)As resistancecontrol layer 8 of thickness 0.5 μm and the p-GaAs (p=4×10¹⁹ cm⁻³) caplayer 9 of thickness 0.03 μm are successively stacked on the upper cladlayer 7.

Moreover, as shown in FIG. 8B, portions of the cap layer 9 andresistance control layer 8 beginning form the laser-emission side endfacet F1 and the laser-reflection side end facet F2 are completelyremoved using an etchant. The width of the removed portion from each endfacet is 25 μm. As a result, portions shown by reference numerals E1 andE2 of the upper clad layer 7 are exposed through removal of theresistance control layer 8 near the laser-emission side end facet F1 andthe laser-reflection side end facet F2 are exposed. Thereafter, as shownin FIG. 8C, the insulating SiN films 32 a and 32 b of thickness 120 nmare deposited in such a manner that these insulating films cover theexposed portions of the upper clad layer 7, the sides of the cap layer 9and resistance control layer 8, and a small portion of the top surfaceof the cap layer 9. At this time, the upper clad layer 7, the resistancecontrol layer 8, and the cap layer 9 are etched into a mesa stripe, andthe SiN films 32 a and 32 b are also formed around the periphery of theetched area. The thickness of the insulating SiN films 32 a and 32 b isnot limited only to 120 nm. The thickness should preferably be 100 nm ormore. If the thickness is less than 100 nm then IL characteristicsdegrade.

Then, as shown in FIG. 8D, the p-electrode 11 is formed in the manner asalready explained in the first embodiment. Similarly, although not shownin this figure, the n-electrode 12 is also formed in the manner asalready explained in the first embodiment. Finally, a coating of ananti-reflection film is created on the laser-emission side end facet F1,and a coating of optical reflection film is created on thelaser-reflection side end facet F2 in the manner as already explained inthe first embodiment.

The etchant T1 explained in the first embodiment is used for selectivelyetching the cap layer 9 and resistance control layer 8. Thep-Al_(0.15)Ga_(0.65)As resistance control layer 8 was completely removedin this etching.

According to the third embodiment, the current non-injection structureis formed by forming insulating SiN films 32 a and 32 b of thickness 120nm that cover the layers exposed in the selective etching. As a result,the current non-injection structures can be surely formed with simplemethod.

The third embodiment explained a case in which the cap layer 9 andresistance control layer 8 in the vicinity of both the laser-emissionside end facet F1 and the laser-reflection side end facet F2 were etchedand insulating SiN films were formed in the vicinity of both thelaser-emission side end facet F1 and the laser-reflection side end facetF2. However, the cap layer 9 and resistance control layer 8 near onlythe laser-emission side end facet F1 may be etched and an insulating SiNfilm may be formed near only the laser-emission side end facet F1.

Moreover, the third embodiment explained a case in which the insulatingfilms that cover the layers exposed in the selective etching is made ofSiN. However, the insulating films may be made of SiO₂ or Al₂O₃.

Furthermore, the third embodiment explained a case in which the etchantT1 is used to selectively etch the cap layer 9 and resistance controllayer 8. However, the etchants T2, T2, and T4 may be used.

Furthermore, it is mentioned in the first to third embodiments that thesemiconductor laser element has a ridge mesa. However, the semiconductorlaser element need not be limited to the one having a ridge mesa. Whenthere exist a lamination comprising in order the p-Al_(x)Ga_(1-x)As,p-Al_(y)Ga_(1-y)As (where x>y>0.2) and p-GaAs layers, then the presentinvention can be applied to selectively etch the p-Al_(y)Ga_(1-y)As andp-GaAs layers.

Furthermore, it is mentioned in the first to third embodiments that theorganic acid is citric acid. However, the organic acid could be anyorganic acid such as malic acid, malonic acid, oxalic acid, tartaricacid.

Furthermore, it is mentioned in the first to third embodiments that thewidth of the etched portion from the end facet is 25 μm. However, thiswidth may be 10 μm or more. When the width is less than 10 μm thecurrent non-injection region does not give proper effect.

Furthermore, it is confirmed with experiments that the curve shown inFIG. 4 is obtained when the volume ratio of the aqueous organic acidsolution to aqueous hydrogen peroxide solution is between 1:1 and 200:1.Thus, although specific figure of the volume ratio of these twosolutions have been mentioned in the first to third embodiments, thevolume ration could be any value between 1:1 and 200:1.

As explained above, according to the semiconductor laser element of oneaspect of this invention, desired layers can be completely etchedwithout harming other layers. Furthermore, an insulating layer which hassubstantially the same thickness as the thickness of the etched layersis formed in the portion from where the layers are etched. As a result,the current non-injection structure can be formed, and it becomespossible to decrease the intensity of light only near side end facet.The layers may be etched only near the laser-emission side end facet, ormay be etched in the vicinity of both the laser-emission side end facetand the laser-reflection side end facet.

According to the semiconductor laser element of another aspect of thisinvention, desired layers can be completely etched without harming otherlayers. Furthermore, an insulating layer which covers only the surfacesof the layers that were exposed due to the etching is formed. As aresult, the current non-injection structure can be formed, and itbecomes possible to decrease the intensity of light only near side endfacet. The layers may be etched only near the laser-emission side endfacet, or may be etched near both the laser-emission side end facet andthe laser-reflection side end facet.

According to the etchant of still another aspect of this invention, theratio of dissolution rate of the first semiconductor layer to the thirdsemiconductor layer is between 10 and 20. As a result, the secondsemiconductor layer can be removed completely.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A semiconductor laser element having alaser-emission side end facet, comprising: a semiconductor substrate; aclad layer stacked directly above said semiconductor substrate or abovesome other layer stacked above said semiconductor substrate; aresistance control layer stacked above said clad layer and whichcontrols a junction resistance between said clad layer and a cap layerstacked above said resistance control layer; said cap layer has a shapesuch that it narrows an electric current flowing in active layers;wherein a portion of said cap layer and said resistance control layer inthe vicinity of said laser-emission side end facet is removed byselective etching, and an insulating layer having a thickness which isthe same as the total thickness of said cap layer and said resistancecontrol layer is formed in the portion that became hollow due toetching; and a top electrode layer stacked above said cap layer and saidinsulating layer.
 2. The semiconductor laser element according to claim1, wherein said semiconductor laser element having a laser-reflectionside end facet, and a portion of said cap layer and said resistancecontrol layer in the vicinity of said laser-reflection side end facet isremoved by selective etching, and an insulating layer having a thicknesswhich is the same as the total thickness of said cap layer and saidresistance control layer is formed in the portion that became hollow dueto etching.
 3. The semiconductor laser element according to claim 1,wherein said insulating layer is made of SiN, SiO₂, or Al₂O₃.
 4. Thesemiconductor laser element according to claim 1, wherein the width fromsaid laser-reflection side end facet of the etched portion of said caplayer and said resistance control layer is 10 μm or more.
 5. Thesemiconductor laser element according to claim 1, further comprising abottom electrode layer stacked below said semiconductor substrate.
 6. Asemiconductor laser element having a laser-emission side end facet,comprising: a semiconductor substrate; a clad layer stacked directlyabove said semiconductor substrate or above some other layer stackedabove said semiconductor substrate; a resistance control layer stackedabove said clad layer and which controls a junction resistance betweensaid clad layer and a cap layer stacked above said resistance controllayer; said cap layer has a shape such that it narrows an electriccurrent flowing in active layers; wherein a portion of said cap layerand said resistance control layer in the vicinity of said laser-emissionside end facet is removed by selective etching, and an insulating filmwhich covers the surface of said clad layer exposed due to the etching,side faces of said resistance control layer and said cap layer, and aportion of top surface of said cap layer; and a top electrode layerstacked above said cap layer and said insulating film.
 7. Thesemiconductor laser element according to claim 6, wherein saidsemiconductor laser element having a laser-reflection side end facet,and a portion of said cap layer and said resistance control layer in thevicinity of said laser-reflection side end facet is removed by selectiveetching, and an insulating film which covers the surface of said cladlayer exposed due to the etching, side faces of said resistance controllayer and said cap layer, and a portion of top surface of said caplayer.
 8. The semiconductor laser element according to claim 6, whereinsaid insulating film is made of SiN, SiO₂, or Al₂O₃.
 9. Thesemiconductor laser element according to claim 6, wherein the width fromsaid laser-reflection side end facet of the etched portion of said caplayer and said resistance control layer is 10 μm or more.
 10. Thesemiconductor laser element according to claim 6, further comprising abottom electrode layer stacked below said semiconductor substrate. 11.The semiconductor laser element according to claim 6, wherein thicknessof said insulating film is 100 nm or more.
 12. An etchant forselectively etching only a first and second layers of a multilayeredsemiconductor laser element comprising at least a third layer and saidsecond and first layers stacked successively on a semiconductorsubstrate, wherein said second layer controls resistance of saidmultilayered semiconductor laser element, wherein said etchant has sucha composition that the ratio of dissolution rate of said firstsemiconductor layer to said third semiconductor layer is between 10 and20.
 13. The etchant according to claim 12, wherein said first layer ismade of p-GaAs, said second layer is made of p-Al_(y)Ga_(1-y)As, saidthird layer is made of p-Al_(x)Ga_(1-x)As where x>y>0.2.
 14. The etchantaccording to claim 12, wherein said etchant is a mixture of organic acidand hydrogen peroxide based mixture.
 15. The etchant according to claim12, wherein the pH of said etchant is between 7.4 and 7.8.
 16. A methodof fabricating a semiconductor laser element, said semiconductor laserelement having a laser-emission side end facet, the method comprisingthe steps of: stacking a semiconductor substrate; stacking a clad layerdirectly above said semiconductor substrate or above some other layerstacked above said semiconductor substrate; stacking a resistancecontrol layer above said clad layer which controls a junction resistancebetween said clad layer and a cap layer stacked above said resistancecontrol layer, said cap layer has a shape such that it narrows anelectric current flowing in active layers; selective etching a portionof said cap layer and said resistance control layer in the vicinity ofsaid laser-emission side end facet; stacking an insulating layer havinga thickness which is the same as the total thickness of said cap layerand said resistance control layer in the portion that became hollow dueto etching; and stacking a top electrode layer above said cap layer andsaid insulating layer.
 17. The method according to claim 16, whereinsaid semiconductor laser element having a laser-reflection side endfacet, the method further comprising the step of: selective etching aportion of said cap layer and said resistance control layer in thevicinity of said laser-reflection side end facet; and stacking aninsulating layer having a thickness which is the same as the totalthickness of said cap layer and said resistance control layer is formedin the portion that became hollow due to etching.
 18. The methodaccording to claim 17, wherein the pH of said etchant is between 7.4 and7.8.
 19. The method according to claim 17, further comprising step ofstacking a bottom electrode layer below said semiconductor substrate.20. The method according to claim 16, wherein the selective etching ofsaid cap layer and said resistance control layer is carried using anetchant for which ratio of dissolution rates of said clad layer to saidcap layer is between 10 and
 20. 21. The method according to claim 16,wherein said insulating layer is made of SiN, SiO₂, or Al₂O₃.
 22. Themethod according to claim 16, wherein the width from saidlaser-reflection side end facet of the etched portion of said cap layerand said resistance control layer is 10 μm or more.
 23. The methodaccording to claim 16, wherein said clad layer is made of p-GaAs, saidresistance control layer is made of p-Al_(y)Ga_(1-y)As, said cap layeris made of p-Al_(x)Ga_(1-x)As where x>y>0.2.
 24. The method according toclaim 16, wherein said etchant is a mixture of organic acid and hydrogenperoxide based mixture.
 25. A method of fabricating a semiconductorlaser element, said semiconductor laser element having a laser-emissionside end facet, the method comprising the steps of: stacking asemiconductor substrate; stacking a clad layer directly above saidsemiconductor substrate or above some other layer stacked above saidsemiconductor substrate; stacking a resistance control layer above saidclad layer which controls a junction resistance between said clad layerand a cap layer stacked above said resistance control layer, said caplayer has a shape such that it narrows an electric current flowing inactive layers; selective etching a portion of said cap layer and saidresistance control layer in the vicinity of said laser-emission side endfacet; stacking an insulating film which covers the surface of said cladlayer exposed due to the etching, side faces of said resistance controllayer and said cap layer, and a portion of top surface of said caplayer; and stacking a top electrode layer above said cap layer and saidinsulating film.
 26. The method according to claim 25, wherein saidsemiconductor laser element having laser-reflection side end facet, themethod further comprising the step of: selective etching a portion ofsaid cap layer and said resistance control layer in the vicinity of saidlaser-reflection side end facet; and stacking an insulating film havinga thickness which is the same as the total thickness of said cap layerand said resistance control layer is formed in the portion that becamehollow due to etching.
 27. The method according to claim 25, wherein theselective etching of said cap layer and said resistance control layer iscarried using an etchant for which ratio of dissolution rates of saidclad layer to said cap layer is between 10 and
 20. 28. The methodaccording to claim 25, wherein said insulating film is made of SiN,SiO₂, or Al₂O₃.
 29. The method according to claim 25, wherein the widthfrom said laser-reflection side end facet of the etched portion of saidcap layer and said resistance control layer is 10 μm or more.
 30. Themethod according to claim 25, wherein said clad layer is made of p-GaAs,said resistance control layer is made of p-Al_(y)Ga_(1-y)As, said caplayer is made of p-Al_(x)Ga_(1-x)As where x>y>0.2.
 31. The methodaccording to claim 25, wherein said etchant is a mixture of organic acidand hydrogen peroxide based mixture.
 32. The method according to claim25, wherein the pH of said etchant is between 7.4 and 7.8.
 33. Themethod according to claim 25, further comprising step of stacking abottom electrode layer below said semiconductor substrate.
 34. Themethod according to claim 25, wherein thickness of said insulating filmis 100 nm or more.